Abstract

There are no conventional lymphatics in the brain but physiological studies have revealed a substantial and immunologically significant lymphatic drainage from brain to cervical lymph nodes. Cerebrospinal fluid drains via the cribriform plate and nasal mucosa to cervical lymph nodes in rats and sheep and to a lesser extent in humans. More significant for a range of human neurological disorders is the lymphatic drainage of interstitial fluid (ISF) and solutes from brain parenchyma along capillary and artery walls. Tracers injected into grey matter, drain out of the brain along basement membranes in the walls of capillaries and cerebral arteries. Lymphatic drainage of antigens from the brain by this route may play a significant role in the immune response in virus infections, experimental autoimmune encephalomyelitis and multiple sclerosis. Neither antigen-presenting cells nor lymphocytes drain to lymph nodes by the perivascular route and this may be a factor in immunological privilege of the brain. Vessel pulsations appear to be the driving force for the lymphatic drainage along artery walls, and as vessels stiffen with age, amyloid peptides deposit in the drainage pathways as cerebral amyloid angiopathy (CAA). Blockage of lymphatic drainage of ISF and solutes from the brain by CAA may result in loss of homeostasis of the neuronal environment that may contribute to neuronal malfunction and dementia. Facilitating perivascular lymphatic drainage of amyloid-beta (Abeta) in the elderly may prevent the accumulation of Abeta in the brain, maintain homeostasis and provide a therapeutic strategy to help avert cognitive decline in Alzheimer's disease.